Study of GaN light-emitting diodes fabricated by laser lift-off techniqueChen-Fu Chu, Fang-I Lai, Jung-Tang Chu, Chang-Chin Yu, Chia-Feng Lin, Hao-Chung Kuo, and S. C. Wang

Citation: Journal of Applied Physics 95, 3916 (2004); doi: 10.1063/1.1651338 View online: http://dx.doi.org/10.1063/1.1651338 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/95/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Improvement of light extraction from GaN-based thin-film light-emitting diodes by patterning undoped GaN usingmodified laser lift-off Appl. Phys. Lett. 92, 141104 (2008); 10.1063/1.2906632 Dichromatic InGaN-based white light emitting diodes by using laser lift-off and wafer-bonding schemes Appl. Phys. Lett. 90, 161115 (2007); 10.1063/1.2722672 Vertical AlGaN deep ultraviolet light emitting diode emitting at 322 nm fabricated by the laser lift-off technique Appl. Phys. Lett. 89, 261114 (2006); 10.1063/1.2424668 Photonic crystal laser lift-off GaN light-emitting diodes Appl. Phys. Lett. 88, 133514 (2006); 10.1063/1.2189159 Fabrication of thin-film InGaN light-emitting diode membranes by laser lift-off Appl. Phys. Lett. 75, 1360 (1999); 10.1063/1.124693

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Study of GaN light-emitting diodes fabricated by laser lift-off techniqueChen-Fu Chu, Fang-I Lai, Jung-Tang Chu, Chang-Chin Yu,Chia-Feng Lin, Hao-Chung Kuo, and S. C. Wanga)

Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan, Republic of China

~Received 8 September 2003; accepted 7 January 2004!

The fabrication process and performance characteristics of the laser lift-off~LLO! GaNlight-emitting diodes~LEDs! were investigated. The LLO-GaN LEDs were fabricated by lifting offthe GaN LED wafer structure grown on the original sapphire substrate by a KrF excimer laser at 248nm wavelength with the laser fluence of 0.6 J/cm2 and transferring it onto a Cu substrate. TheLLO-GaN LEDs on Cu show a nearly four-fold increase in the light output power over the regularLLO-LEDs on the sapphire substrate. High operation current up to 400 mA for the LLO-LEDs onCu was also demonstrated. Based on the emission wavelength shift with the operating current data,the LLO-LEDs on Cu show an estimated improvement of heat dissipation capacities by nearly fourtimes over the light-emitting devices on sapphire substrate. The LLO process should be applicableto other GaN-based LEDs in particular for those high light output power and high operation currentdevices. ©2004 American Institute of Physics.@DOI: 10.1063/1.1651338#

I. INTRODUCTION

GaN-based wide-band-gap semiconductors are widelyused for optoelectronic devices, such as blue light-emittingdiodes~LEDs! and laser diodes~LDs!. These devices weregrown heteroepitaxially onto dissimilar substrates, such assapphire and SiC, because of difficulties in the growth ofbulk GaN. The sapphire is the most commonly used substratebecause of its relatively low cost. However, due to the poorelectrical and thermal conductivity of sapphire substrate, thedevice process steps are relatively complicated comparedwith other compound semiconductor devices. Therefore,GaN optoelectronics devices, fabricated on an electricallyand thermally conducting substrate by separating sapphiresubstrate, are most desirable. The separation of hydride va-por phase epitaxy-grown 2 in. GaN wafer from the sapphiresubstrate was demonstrated1 by using the laser lift-off~LLO!technique. The pulsed high-power UV laser of the third har-monic of aQ-switched Nd:YAG laser with a 355 nm wave-length was used to irradiate through the transparent sapphiresubstrate to lift off the GaN film. The physical process re-sponsible for the lift off appears to be dominated by the rapidthermal decomposition of GaN near the sapphire interface.Recently, the LLO technique has been used to fabricate free-standing InGaN LEDs and LDs.2–4 These includep-side upGaN LEDs with Ti/Al asp and n contact, thep-side downInxGa12xN LEDs on Si substrates with Pd–In as thep-GaNcontact and bonding metal, andp-side up InGaN laser diodeson copper substrates. However, in Refs. 2–4, differentp-GaN contact metals were used for either the conventionalLEDs and LLO-LEDs or the LLO-LEDs were mounted inthe same transferred substrate, which makes the performancecomparison relatively difficult. In our recently report,5 wecompared the performance ofp-side up andp-side down

LLO-LEDs on a copper substrate with the samep-GaN con-tact metals, and demonstrated a superior performance of thep-side down configuration of GaN LEDs over thep-side upLEDs. The result suggests that thep-side down LLO-LEDscan enhance the light output power, operate at high current,and increase the heat capacity of GaN-based LEDs. In thisarticle, we report the detail investigation results of the GaNLLO process and the establishment of GaN LLO conditions,the demonstration of the GaN light emitting devices withhigh current operation.

II. LASER LIFT-OFF SETUP AND PROCESSCONDITIONS

Figure 1 shows the schematic diagram of the setup forthe conducting LLO experiment. A KrF excimer laser~Lambda Physick LPX210! at wavelength ofl5248 nmwith pulse width of 25 ns was used for LLO. The laser out-put energy can be varied from 10 nJ to 25 mJ. The laserbeam is reshaped and homogenized using a special opticalsystem to form a highly uniform@65% root-mean-square~rms!# beam profile of 12312 mm2 after the mask plane. Abeam splitter then splits the laser beam into a LLO beam anda monitor beam. The LLO processing beam passed through aprojection system of 103 with a 0.2 numerical aperture, andthen focuses onto the sample with a square spot size of 1.231.2 mm2. The monitor beam is incident on a beam analyzerfor real-time monitoring of the laser beam quality. Thesamples were placed on the top of working station which canbe moved 1.2 mm step by step to scan a typical sample sizeof 2 cm32 cm by using the computer-controlled stepper mo-tor. The charge coupled device camera was used toin situmonitor the LLO process. For establishing LLO conditionsfor GaN, the effect of laser fluence on the ablation of GaNmaterials under a different environment was investigatedfirst. A bare undoped GaN sample of 14.9mm thickness wasirradiated with the laser beam under two pressure conditions:

a!Author to whom correspondence should be addressed; electronic mail:scwang@cc.nctu.edu.tw

JOURNAL OF APPLIED PHYSICS VOLUME 95, NUMBER 8 15 APRIL 2004

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One atmosphere pressure and 1023 Torr. The laser fluencewas varied from 0.2 to 1.0 J/cm2 at a constant number ofpulses. The laser irradiation causes the decomposition ofGaN into gaseous nitrogen and gallium droplets, followingthe equation:

GaN⇔Ga11

2N2~g! .

The GaN sample after laser irradiation tends to show somematerial residues, such as Ga and Ga oxide. These residueswere then cleaned up by a dilute acid solution, such as HClor H2SO4 /H2O2, before the measurement of the etcheddepth.

Figure 2 shows the rate of removable of GaN, or etchingrate, as a function of laser fluence for the two different pres-sure conditions. The etching rates increase with increasinglaser fluence under both conditions. At the incident laser flu-ence of 1.0 J/cm2, the etching rate was about 35 nm/pulse,and 60 nm/pulse for one atmosphere pressure and 1023 Torr,respectively.

According to the chemical kinetic theory,6 a system instable chemical equilibrium subjected to the influence of anexterior force tends to cause a variation in its pressure. For aGaN sample irradiated under the low pressure of 1023 Torr,which is lower compared with the one atmosphere condition,

the decomposition rate of GaN should be higher than that ofthe sample placed in one atmosphere condition. Thus, ahigher etching rate was obtained in a low-pressure condition.Based on the laser etching of GaN sample, we obtained thethreshold laser fluence for ablation of GaN surface to beabout 0.3 J/cm2.

From the etching rate and the incident laser fluence re-lationship, the GaN material absorption coefficient at a KrFlaser wavelength can also be estimated based on the widelyaccepted formula of Beer’s law:7

d5S 1

a D lnS Ei

EthD ,

whered is the etched depth per pulse,a is the absorptioncoefficient,Ei is the incident laser fluence, andEth is thethreshold laser fluence for material removal. By plotting theetching rate versus logarithm of incident fluenceEi , we ob-tained an absorption coefficient of about 2.5(62.0)3105 cm21 from the slopes of the plot. This value is inagreement with the reported data for GaN obtained from theoptical measurement method.8

The dependence of the etched depth on the numbers ofpulses at a higher laser fluence of 3.75 J/cm2 was also inves-tigated to establish the optional LLO condition. The samplewas etched under an atmosphere condition with a differentnumber of pulses. Figure 3 shows the etched depths as afunction of pulse numbers. The etched depth increases lin-early with the numbers of pulses. This dependence is similarto the reported elsewhere.9 From these data, the higher etch-ing rate, as high as 82 nm/pulse, was obtained at a higherlaser fluence. Figure 4 shows a scanning electro microscopy~SEM! picture of the etched sidewall. The etched surfacemorphology was also obtained by atomic force microscopy~AFM! measurement in contact mode operation with a scanarea of 5mm2. Figure 5 shows the typical AFM images forthe samples under these two different conditions. The rmsroughness of the surface morphology is around 4–14 nm forthe sample processing in the atmosphere pressure condition,and around 30–36 nm for the sample processing under a lowpressure condition. These results show that the decomposi-

FIG. 1. The schematic diagram of laser etching and LLO process setup.

FIG. 2. The rate of removable of GaN or etching rate as a function of laserfluence for the two different pressure conditions.

FIG. 3. The etching rate as a function of pulse numbers at the laser fluenceof 3.75 J/cm2 under the atmosphere condition.

3917J. Appl. Phys., Vol. 95, No. 8, 15 April 2004 Chu et al.

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tion of GaN material under the atmosphere pressure condi-tion has a relatively better surface morphology. Therefore,the GaN LLO was conducted process under the atmosphereenvironment.

The relation between the decomposition temperature atthe irradiated surfaceT, and the absorbed irradiationI a formaterial heating can be expressed as:10

T~ t !5I a

j

KAp~12R!,

wherej52(Dt)1/2 is the diffusion length, and the thermaldiffusivity D5K/Cpr, K is the thermal conductivity,Cp isthe heat capacity,r is density,t is the pulse duration, andR isthe reflectivity. For GaN material, the decomposition tem-perature~T! is about 900 °C–1000 °C,11–13 K51.3 W/cm K,Cp59.75 cal/mol K, r56.11 g/cm3, t525 ns, andR50.3.14–16 Using these values, the absorbed laser fluencerequired for decomposition of GaN is about 0.3 J/cm2, whichis in agreement with the experiment value.

The LLO process was first conducted using an undopedGaN sample grown on a sapphire substrate. The back side ofsapphire was first polished by various sizes of diamond pasteranging from 3mm to 1 mm, then cut to a sample size of 1cm31 cm. The sample was bonded to a Si wafer using cy-anoacrylate (C6H7NO2)-based ester adhesive forming asapphire/u-GaN/epoxy/Si structure. The sample was placed

on the top of the workstation in air. Figure 6 shows the LLOprocess sequence for the fabrication of freestanding undopedGaN. The pulsed KrF excimer laser with a spot size of 1.2mm31.2 mm was directed through the back side of the pol-ished transparent sapphire substrate. Since the attenuation ofthe KrF excimer laser through the 0.5 mm thick sapphiresubstrate is approximately 20%–30%~Ref. 17! and the re-flection of the interface of sapphire/GaN at 248 nm is ap-proximately 20%,9 the incident laser fluence was set to avalue of 0.6 J/cm2, corresponding to a laser fluence of about0.3 J/cm2 at the interface. With the incident fluence of 0.6J/cm2, the metallic silver color with size of 1.2 mm31.2 mmclearly appeared in the interface of GaN and sapphire withthe ejection of some dustlike particles from the edge ofsample, indicating the decomposition of the Ga interfaciallayer between GaN and sapphire. By heating the irradiatedsamples at the Ga melting point of 30 °C, the GaN film wasthen easily separated from the sapphire substrate and trans-ferred onto the Si supported carrier. The metallic Ga residueswere removed to complete the LLO process.

The LLO GaN sample was characterized by using scan-ning electron microscopy~SEM!, AFM, x-ray rocking curve~XRC!, and photoluminescence~PL! spectrum measure-ments. Figure 7 shows a typical cross-sectional SEM micro-graph of the LLO GaN sample. The structure of GaN/epoxy/Si was clearly depicted. The thickness of LLO-GaN

FIG. 4. The SEM picture of the etched sidewall.

FIG. 5. The typical AFM images for the samples under these two differentconditions:~a! The atmosphere condition and~b! low pressure of 1023 Torr.

FIG. 6. The process scheme for fabrication of the LLO undoped GaN.

FIG. 7. A typical cross-sectional SEM micrograph of 14.9-mm-thick un-doped sample.

3918 J. Appl. Phys., Vol. 95, No. 8, 15 April 2004 Chu et al.

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film was measured around the value of 14.9mm indicatingthe reduction in thickness is relatively small after the LLOprocess. Figure 8 shows typical AFM images of the LLO-GaN film surface. Uneven and whiskerlike micropole struc-tures were formed as the result of the lift-off process. Therms values of the GaN surface roughness were around 12nm, which is consistent with the results of the GaN-etchedsurface for the sample etching in the atmosphere pressurecondition. The rms value of 12 nm compared to the value of0.3 nm for the sample before LLO was larger due to thethermal decomposition which occurred in the interface ofGaN and sapphire. Figure 9 shows the XRC spectrum of theGaN sample before and after lift off. The full width at halfmaximum~FWHM! value of the LLO film is about 528 arcs,which is slightly larger than that of the regular GaN film of412 arcs. The increase in the FWHM of the rocking curvemaybe due to the thermally induced lattice disorder carriedby the laser irradiation.18 Figure 10 shows the PL spectrumof the GaN sample before and after the LLO process. Theemission peak of LLO-GaN is at 364 nm, which is slightlyredshifted compared to the emission peak before LLO. A

similar redshift was reported previously18 and attributed tothe induced localized defects in the GaN/sapphire interfacecaused by the LLO process.

III. FABRICATION OF LASER-LIFT OFF-GaNLIGHT-EMITTING DEVICES

The LLO process was used to fabricate freestandingGaN LED on copper substrate. The copper substrate wasselected for its good electrical and high thermalconductivity.19 The LED wafer structure was grown by met-alorganic chemical vapor deposition on~1000! sapphire sub-strate. The LED structure consists of a 25-nm-thick GaNlow-temperature buffer layer, a 1.5-mm-thick undopedu-GaN layer, a 1.5-mm-thick highly conductiven-type GaNlayer, a multiple quantum well~MQW! region consisting offive periodsu-GaN 2/5-nm-thick InGaN/GaN MQWs, and a0.3-mm-thick p-type GaN layer.

The fabrication process of the LLO-LEDs on a Cu sub-strate is shown in Figs. 11~a!–11~h!. The process involvesthe LLO of the LED wafer process first followed by the

FIG. 8. A typical AFM image of the LLO undoped GaN film surface.

FIG. 9. The XRC spectrum of the undoped GaN before and after LLO.

FIG. 10. The PL spectrum of the undoped GaN before and after LLO.

FIG. 11. The schematic diagram of the fabrication process for the LLO-LEDs on Cu substrate:~a! p-type metal deposition,~b! Cu substrate bond-ing, ~c! laser processing.~d! separation,~e! etching of u-GaN layer, ~f!300-mm-square mesa formation and device isolation,~g! n-type contactdeposition, and~h! freestandingp-side down LLO-LEDs on Cu.

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fabrication of LEDs on a Cu substrate. The GaN LED wafersample with size of about 1 cm31 cm was deposited withp-contact metallization using Ni/Pd/Au~20 nm/20 nm/100nm!20 as thep-GaN contact and the back side of sapphiresubstrate was polished. Then, the sample was annealed inoxygen at 550 °C for 5 min first to form thep-type ohmiccontact. The sample was then bonded onto an indium-coatedCu substrate at 200 °C to form a structure of sapphire/GaNLED/Ni/Pd/Au/In/Cu. In this process, the Ni/Pd/Aup-typecontact also serves as the bonding material without using anyother bonding substance. The bonded structure was then sub-jected to the LLO process as described earlier to form anu-GaN/n-GaN/MQW/p-GaN structure on a Cu substrate.Then, theu-GaN was etched away by inductively coupledplasma reactive ion etching~ICP/RIE! to expose then-GaNlayer. The typical rough and uneven surface of theu-GaNafter the LLO process was also evened out by the ICP etch-ing process to form a relatively smooth surface for then-contact formation. Then, a square mesa of 300mm3300mm was created by ICP/RIE for current isolation purposes.Finally, a Ti/Al with a diameter of 100mm circular pad wasdeposited as then-type contact on the center of square mesawithout any other semitransparent contact layer. The com-pleted top and side views of the LLO-LEDs on Cu are shownin Fig. 12. It has a low resistivityn-GaN top layer whichfacilitates better current spreading21 without the need for anadditional semitransparent ohmic contact metallization typi-cal of the regular LEDs on sapphire substrate. The LLO-LEDon Cu has a lager emission area of about 90% compared tothe entire mesa range~300 mm3300 mm!.

IV. PERFORMANCE OF LASER LIFT OFF-LIGHT-EMITTING DIODES

In order to compare the performance of LLO-LEDs onCu with regular GaN LEDs on a sapphire substrate, regularGaN LEDs on sapphire were also fabricated using the samewafer structure. Figure 13 shows the scheme of the regularLEDs, which has a Ni semitransparent layer and the samemetalp- andn-contact metallization as the LLO-LEDs. Theregular GaN LEDs on sapphire has an emission area of about50% compared to the entire mesa range~300mm3300mm!.

Figure 14 shows the light output power–current–voltage

(L – I –V) characteristics under low current continuous-wave~cw! operation conditions for the LLO-LEDs on Cu, and theregular LEDs on sapphire. The voltage at 20 mA is about 4.2V for the regular LEDs on sapphire and 6.5 V for the LLO-LEDs on Cu. The higher operating voltage of the LLO-LEDson Cu could be caused by the degradation in thep contactafter the bonding process. Nevertheless, the light outputpower of the LLO-LEDs on Cu was about four times largerthan that of regular LEDs on sapphire at the operated currentof 20 mA. The increase in the output power could be due toseveral factors. First, the effect of the light-emitting area ofthe LLO-LEDs on Cu is about 1.8 times larger than that ofthe regular LEDs on sapphire. Second, the GaN/Ni interfacehas a reflectivity of about 37%~Refs. 22–24! at a LEDwavelength of 470 nm which is higher than that of the GaN/sapphire interface of 2%. The light output has about a 34%enhancement for the LLO-LEDs on Cu. Third, the LLO-LEDs on Cu withn-side up configuration without a semi-transparent metal has about a 1.6 times higher light outputpower than that of the regular GaN LEDs on sapphire due tono light absorption by the semitransparent contact metal. Fi-nally, according to Ref. 21, the GaN LEDs with an-side upconfiguration tend to have higher light output power com-pared to the regular GaN LEDs on sapphire due to no current

FIG. 12. The completed top and side view of the LLO-LEDs on Cu.

FIG. 13. The typical top and side views of the regular GaN LED on sap-phire substrate.

FIG. 14. TheL – I –V characteristics for the LLO-LEDs on Cu, and theregular LEDs on sapphire.

3920 J. Appl. Phys., Vol. 95, No. 8, 15 April 2004 Chu et al.

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crowding effect. Furthermore, the LLO-LEDs on Cu requireno additional semitransparent metal layers, which simplifiesthe fabrication process.

Figure 15 shows the comparison of the light outputpower–current (L – I ) characteristics for the LLO-LEDs onCu, and the regular LEDs on sapphire under high current cwoperation conditions. The light output power of the regularLEDs on sapphire increased with increasing current up toabout 225 mA, saturated at around 225 mA, and graduallydegraded when the operation current exceeded 270 mA. Onthe other hand, the light output power of the LLO-LEDs onCu increased with increasing current up to about 225 mA andmaintained the same power level after exceeding 225 mA,and was still operational up to 400 mA; suggesting superiorheat dissipation with the Cu substrate and also allowinghigher current operation with higher light output.

Figure 16 shows the variation of LED emission peakwavelength for the regular LEDs on sapphire and the LLO-LEDs on Cu under high current cw operation conditions. Thepeak wavelength of regular LEDs on sapphire showed a rela-

tively larger redshift of about 9 nm from a low current of 20mA to a high current of 250 mA. While, the peak wavelengthof the LLO-LEDs on Cu showed only a slight redshift ofonly about 2–3 nm in the same current range. Using thereported wavelength driftDl with temperatureDTj value of0.06 nm/°C~Ref. 25! for a GaN MQW LED, we estimatedthe junction temperature increase based on the wavelengthshift of the regular LEDs on sapphire to be about 150 °Ccorresponding to a temperature increase rate of 0.65 °C/mA.While for the LLO-LEDs on Cu, the junction temperatureincrease is about 40 °C corresponding to a temperature in-crease rate of 0.17 °C/mA which is about four times lowerthan that of the regular LEDs on sapphire. These results in-dicate the LLO-LEDs on a copper substrate do provide amuch better heat dissipation capability than the regular LEDson sapphire substrate, and allows higher current operation.

V. CONCLUSION

In conclusion, we have fabricated GaN LLO-LEDs onCu by using a KrF excimer laser at 248 nm with the laserfluence of 0.6 J/cm2 to separate the GaN LED wafer structurefrom the sapphire substrate first and then transfer the LLO-LED wafer structure onto a Cu substrate. The LLO-LEDs onCu showed a substantial increase in light output power ofabout four fold over that of the regular LEDs fabricated onthe sapphire without degradation in the output light intensity.From the wavelength shift with the operating current data,the LLO-LEDs on Cu showed a four times greater estimatedimprovement of in heating dissipation capacity over theregular LEDs on sapphire. The LLO-LEDs on Cu with ap-side down configuration in particular should be suitable forhigh light output power, and high operation current GaN-based optoelectronic devices.

ACKNOWLEDGMENTS

This work was supported in part by the National ScienceCouncil of Republic of China~ROC! in Taiwan under Con-tract No. NSC 92-2215-E-009-015 and by the Academic Ex-cellence Program of the Ministry of Education of ROC underContract No. 88-FA06-AB.

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